Referring to FIG. 1, there is shown a partial simplified side view shown partially in section of a conventional cathode ray tube (CRT) 10 such as of the monochromatic (single beam) type. CRT 10 comprises a multi-electrode electron gun 11 disposed within a sealed glass envelope 13, a magnetic deflection yoke 18 disposed outside the glass envelope, and a display screen 14 having disposed on the inner surface thereof a phosphor layer 16. A heated cathode K emits energetic electrons into a beam forming region (BFR) in a narrow neck portion 13a of the glass envelope 13. BFR is comprised of a G.sub.1 control electrode, a G.sub.2 screen electrode, and a facing portion of a G.sub.3 electrode. Each of the aforementioned G.sub.1, G.sub.2 and G.sub.3 electrodes, or grids, as these two terms are used interchangeably herein, as well as a G.sub.4 electrode described below, is maintained at a designated voltage, or potential, as these two terms are used interchangeably in the following discussion, by means of one or more power supplies, which are not shown in the figure for simplicity. The thus formed electron beam 12 is directed along an axis A--A' toward the CRT's display screen 14. An electrostatic field formed by the G.sub.1, G.sub.2 and G.sub.3 electrodes forms the energetic electrons into a beam and exerts a first focus effect on the beam. Electron gun 11 further includes a main focus lens which includes the G.sub.4 electrode and a facing portion of the G.sub.3 electrode. The main focus lens applies a greater electrostatic focus field to the electron beam 12 for focusing it on the display screen 14.
A high voltage typically on the order of 25 kV is introduced into the CRT 10 by means of an anode button 30 extending through envelope 13. An anode conductor (not shown in the figure for simplicity) generally in the form of a thin conductive coating disposed on an inner surface of the glass envelope 13 provides the high voltage to an anode grid G.sub.4 via a support cup 20 for accelerating the electrons in the beam to a high energy before reaching the display screen 14. It is the high energy of the electrons in the beam which excites the phosphor layer 16 to provide a visual image on the display screen 14. Each of the aforementioned electrodes is coaxially disposed about the electron beam axis A--A' and includes one or more apertures aligned with the beam axis A--A' for allowing electron beam 12 to be directed onto display screen 14. Each of the aforementioned electrodes is typically attached to a support arrangement such as a pair of glass rods, which also are not shown in the figure for simplicity. The support, or convergence, cup 20 is also typically attached to the high voltage end of the G.sub.4 electrode for maintaining the electrode securely in position in CRT 10 and centered on the electron beam axis A--A'. Bulb spacers 22 extending from the support cup 20 provide support and electrical contact with the anode voltage. The G.sub.3 electrode is frequently disposed within an element exhibiting high magnetic permeability to shield the electron beam within the CRT's main focus lens from the magnetic deflection field of yoke 18.
The electron gun's main focus lens is therefore typically comprised of the G.sub.3 and G.sub.4 electrodes and has a focal point 26 located on axis A--A' intermediate these two charged electrodes. The main focus lens formed of electrodes G.sub.3 and G.sub.4 also has an equivalent lens size, which is relatively small in diameter for the typical electron gun 11 shown in FIG. 1 because of the relatively small diameter of these focus electrodes. The small equivalent lens diameter increases spherical aberration of the electron beam. After the electron beam is focused by the main focus lens, it then passes through a deflection region formed by magnetic deflection yoke 18 disposed about the CRT's envelope 13. Deflection yoke 18 typically is comprised of a toroidal ferrite core about which is wound a current carrying conductor, or conductors, for establishing a time-varying magnetic field within the CRT 10 for deflecting electron beam 12 across the inner surface of the display screen 14 in a raster-like manner. The deflected electron beam is represented in dotted-line form as element 12' in FIG. 1. In a conventional CRT, the electron beam is therefore first electrostatically focused and then magnetically deflected across the display screen 14. A beam deflection center is formed in the magnetic deflection region such as on a deflection center line D--D' shown in FIG. 1, with its location depending upon the location of the deflection yoke 18 and the size and shape of the yoke's core and conductive wire arrangement. From the figure it can be seen that the deflection center line D--D' is disposed forward of the main focus lens comprised of the G.sub.3 and G.sub.4 electrodes. In addition, the main lens focal point 26 is displaced from the magnetic deflection region and the deflection center line D--D'. This spatial separation of the CRT's focus and deflection regions is one factor which determines the CRT's length.
One problem with the prior art CRT 10 shown in FIG. 1 arises from the sequential focusing and deflection of the electron beam 12. When the electron beam 12 reaches the deflection center line D--D', the electrons have been accelerated to a high energy by the anode voltage V.sub.A which is typically applied to the G.sub.4 electrode. Because the amount of deflection for a given magnetic field is inversely proportional to the square root of electron beam voltage, a large magnetic field is required to deflect the beam. This generally requires a larger deflection yoke or increased current in the yoke windings which gives rise to thermal dissipation problems and requires a larger yoke power supply. Beam deflection sensitivity also is reduced at high beam energies. High deflection sensitivity is particularly important in the current high resolution CRTs with higher deflection frequencies. In order to accommodate these faster deflection rates, Litz wire in the form of a bundle of twisted wires is frequently used to provide a greater surface area in taking advantage of the increased skin effect of these types of conductors. Unfortunately, Litz wires are substantially more expensive than a strand of conventional copper wire and of limited commercial value in consumer-type CRTs.
The present invention addresses the aforementioned limitations of the prior art by providing a deflection lens for an electron gun in a CRT which allows for simultaneous and co-located focusing and deflection of the CRT's electron beam. By positioning the electron beam's deflection center within the focal point of the CRT's main focus lens, increased beam deflection sensitivity is realized, the length of the CRT as well as the diameter of its neck portion may be reduced, and electron beam space charge effect and focus lens spherical aberration are reduced for improved video image quality.